Autism spectrum disorders (ASD) and specific language impairment (SLI) are both complex disorders that involve primary impairments in language that are usually diagnosed on the basis of emerging behavioral symptoms during the toddler or preschool period. Twin and family studies have demonstrated high heritability estimates for both ASD and SLI and there is evidence from these studies for overlap among these disorders, suggesting partially shared etiology. For children diagnosed with ASD and SLI there is considerable variability in outcomes, especially in the severity of language and social-communication impairments. The main goal of the proposed exploratory research program is to develop a new methodological approach for identifying the earliest predictors of risk and early course of language and communication development for ASD and SLI between the ages of 6 and 12 months. A longitudinal prospective study of infants at high risk for ASD and SLI is proposed. 100 high risk infant siblings of probands with either ASD (N=50) or SLI (N=50), as well as low-risk sibling controls (N=30) will be recruited, and followed from 6 to 18 months of age, at which time provisional assessments of ASD and language delay will be conducted. Our goal is to acquire dense home-based diary records of the infants' development using novel structured parent data collection methods, including detailed diaries and video-tapes of the infants that will be coded for the emergence of early vocal, language and social-communicative behaviors. These diaries will be complemented by laboratory-based standard assessments at 6, 12 and 18 months. Analyses will focus on comparing the two high-risk sibling groups to one another and to the controls on the timing of emergence of key developmental milestones and the presence of unusual behaviors. The findings from this research program will lead to the development of new clinically useful methods for diagnosing and assessing ASD and SLI in infancy that could be implemented in community settings. Early diagnosis of high risk for these disorders is considered a crucial step in improving the prognosis for children with ASD and SLI, because this will open the doors for early entry into interventions that are known to lead to better outcomes. [unreadable] [unreadable] [unreadable]